Allergic disorders are induced by antigen-antibody reactions. When an individual has been immunologically primed or sensitized, further contact with antigen can lead not only to secondary boosting of the immune response but can also cause tissue-damaging reactions, i.e., allergic disorders. The mechanism of pathogenesis of allergic disorders is presently believed as follows:
An individual produces antibodies after exposure to pathogenic antigen. Secondary antigen exposure causes antigen-antibody reaction and the formed antigen-antibody complexes deposit on the tissues, and chemical mediators are released from sensitized cells. Then these mediators and/or the deposited antigen-antibody complexes damage tissues.
Pathogenic antigens are xenogenic antigens (inhaled allergen, food allergen, drugs and so on), allogenic antigens and autologus antigens which are denatured autologus components of tissues or organs, and act as foreign substances.
So-called allergic disorders may be classified into four types;
(1) Type I allergy (anaphylactic-type), in which the antigen reacts with a specific class of antibody bound to mast cells or circulating basophils through a specialized region of the antibody. This leads to degranulation of the cells and release of vasoactive mediators;
(2) Type II allergy (cytotoxic-type), in which the antibodies on the cell surface bind to an antigen and cause several reactions such as opsonic or immune phagocytosis of the cell, and cell lysis by the action of the complement system;
(3) Type III allergy (Arthus type; immune complex mediated), in which a complex is formed between antigen and humoral antibody and causes activation of the complement system, platelet aggregation, microthrombi formation, and so on;
(4) Type IV allergy (cell-mediated or delayed-type), in which thymus-derived lymphocytes (T cells) with specific receptors are stimulated by antigens and release mediators. In case of tissue rejection, these lymphocytes transform to kill certain cells with the histocompatibility antigen of the graft.
Among the allergic reactions, Types I, III and IV allergies participate in, for example, bronchial asthma and each of these reactions is considered independently, or in combination to cause these disorders. The mechanism of induction of allergic disorders is considered as follows: an antigen which enters an organism is treated by macrophages and the immunological information is transmitted to the T cell-B cell system. The B cells which have received the information produce anibody (IgE antibody is mainly produced in Type I allergy and IgG antibody in Type II or Type III allergy). IgE antibody binds to basophils in circulation or to mast cells in the tissues, thereby establishing the sensitized state. Hereafter, the same antigen which enters the sensitized organism binds with the antibody on these cells and releases chemical mediators, such as histamine, or slow reacting substances of anaphylaxis (SRS-A). The chemical mediators thus released induce allergic symptoms such as erythema, edema, or increase of glandular secretion caused by contraction of smooth muscles and increase of capillary permeability. On the other hand, IgG-antibody binds polymorpho-nuclear leukocytes to achieve sensitization, and SRS-A as a chemical mediator is thought to be secreted.
Agents for treatment of allergic disorders can achieve their therapeutic purpose by inhibiting any step in the above-mentioned processes. For example, xanthine derivatives, .beta.-adrenergic stimulants (.beta.-stimulants) or corticosteroids are used for treatment of bronchial asthma. However, unfavorable adverse reactions have often been observed in these drugs. For example, palpitation and tachycardia are reported in patients receiving xanthine derivatives and .beta.-stimulants. Furthermore, corticosteroids cause adverse reactions such as peptic ulcer and complication of bacterial infection. Anti-histamine agents are not effective for bronchial asthma; these agents sometimes make the asthma even worse by making it difficult to expectorate tracheal secretions.
Immune complex diseases, represented by rheumatoid arthritis, systemic lupus erythematosus (SLE) and lupus nephritis, as implied by the name, are diseases which are induced by complexes of antigens with antibodies, i.e., immune complexes, and are type III allergies. Although the mechanism of occurence of these diseases is complicated and has many points which are left unclear, it is generally believed to follow the course described below.
When bacterial or viral infections damage tissues, antibodies are produced against newly formed autoantigens or virally infected cells to form immune complexes. Since these immune complexes activate the complement system and platelets, vasoactive substances such as histamine and serotonin are released and the permeability of the blood vessels is increased. Then, the immune complexes in circulation enter and deposit along the basement membrane of the vessel wall whose permeability has been increased. Where the immune complexes have deposited, polymorphonuclear leukocytes are gathered by the action of the leukocyte chemotactic factors which have been formed by the action of the complement to the deposited immune complexes. The polymorphonuclear leukocytes, reacting with the immune complexes, release various tissue-damaging substances such as cathepsins D and E, collagenase, elastase and permeability factors, and these substances eventually damage the tissue. In patients with immune complex diseases such as SLE, levels of the complement in the serum are generally low and aggravation of the disease conditions is closely correlated with the decrease of the complement levels. This decline is thought to be due to plentiful consumption of the complement at the site of the reaction between antigens and antibodies taking place such as in kidneys and blood vessels. Further, the immune complexes also are related to blood coagulation systems, and it is believed that the immune complexes cause serious symptoms through diverse mechanism for example by acceleration of fibrinoid deposition on the damaged tissues.
Today, there are several kinds of agents for the treatment of immune complex diseases: immunosuppressive agents such as steroids which suppress activation of the immune system, anti-inflammatory agents which reduce local inflammations and pain, or anticoagulative agents and antiplatelet agents which serve to improve abnormalities of the coagulation-fibrinolysis system in the blood vessels. However, these agents are not satisfactorily effective and are associated with strong adverse reactions. Thus, development of a medicine which is safe and highly effective in the treatment of the diseases is strongly desired.
Furthermore, many agents have been developed for the treatment of malignant tumors.
The anti-tumor agents are classified roughly into two types. The first includes so called cytotoxic drugs which directly suppress the growth of tumors. The second includes the drugs which indirectly control the growth of tumors by activating the immunological protective functions of the host. However, the former do not exhibit sufficient selective cytotoxicity against the tumor cells and are toxic also against normal cells, whereby the total amount of the agent which can be used is considerably limited. On the other hand, the latter, i.e., immunopotentiators, are generally safely used, less frequently exhibiting unfavorable adverse reactions compared to the former. However, tumors are originated from the normal cells of the patient, so that the tumor may not sufficiently be recognized as a foreign substance. Therefore, some immunopotentiators have an essential problem that they do not elicit sufficient anti-tumor effect.